Billy:
Onset is a big marina. Walk into Onset but do not expect too much. Another choice would be the one operated by Mass Maritime just behind the school at the entrance to Buttermilk Bay. Walk into Buzzards Bay, but again, not much to do.
With a 35' twin engine, you will not have any problems with the currents in the canal. Two different tides so it really doesn't matter which one you pick. Speed is limited to 10 mph (not knots)and they monitor it by radar. When the tides are opposite, there can be 2-3 feet of standing water about half-way through. 100 feet on either side, it is flat calm.
Constitution Marina is great! If you get an inside slip, all the better, but it can be a tight fit. They have dock hands to help. Walk over the North Washington Street Bridge to Boston's North End. Restaurants and pastry shops second to none.
Pickering Wharf is a very short walk to downtown. Stay in the channel as you pass the power plant. The floating buoys shift to dolphins and will bring you to Derby Wharf. If there are no moorings in a spot, it is because there is no water at low tide.
On the return, I would discourage you from running down Buzzards Bay in the afternoon. Typically, the wind comes up in the afternoon from the southwest. The bay is funnel shaped from the southwest. With a nice long fetch, all that water has no where to go but up! Of course, you might get lucky.
If you can, I would suggest Cuttyhunk Harbor on the way up (the "marina" is owned by the town). You can then run up to Sandwich Basin at the east end of the canal, refuel and make the run to Boston or Salem. Sandwich to Salem is about a 50 NM run. Boston is about the same to the marina. On the way home, stop at Onset to get a fresh start in the morning before the wind picks up.
Sounds like a great trip.
Gene